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<H3>displace_atoms command 
</H3>
<P><B>Syntax:</B>
</P>
<PRE>displace_atoms group-ID style args keyword value ... 
</PRE>
<UL><LI>group-ID = ID of group of atoms to displace 

<LI>style = <I>move</I> or <I>ramp</I> or <I>random</I> or <I>rotate</I> 

<PRE>  <I>move</I> args = delx dely delz
    delx,dely,delz = distance to displace in each dimension (distance units)
  <I>ramp</I> args = ddim dlo dhi dim clo chi
    ddim = <I>x</I> or <I>y</I> or <I>z</I>
    dlo,dhi = displacement distance between dlo and dhi (distance units)
    dim = <I>x</I> or <I>y</I> or <I>z</I>
    clo,chi = lower and upper bound of domain to displace (distance units)
  <I>random</I> args = dx dy dz seed
    dx,dy,dz = random displacement magnitude in each dimension (distance units)
    seed = random # seed (positive integer)
  <I>rotate</I> args = Px Py Pz Rx Ry Rz theta
    Px,Py,Pz = origin point of axis of rotation (distance units)
    Rx,Ry,Rz = axis of rotation vector
    theta = angle of rotation (degrees) 
</PRE>
<LI>zero or more keyword/value pairs may be appended 

<PRE>  keyword = <I>units</I>
    value = <I>box</I> or <I>lattice</I> 
</PRE>

</UL>
<P><B>Examples:</B>
</P>
<PRE>displace_atoms top move 0 -5 0 units box
displace_atoms flow ramp x 0.0 5.0 y 2.0 20.5 
</PRE>
<P><B>Description:</B>
</P>
<P>Displace a group of atoms.  This can be used to move atoms a large
distance before beginning a simulation or to randomize atoms initially
on a lattice.  For example, in a shear simulation, an initial strain
can be imposed on the system.  Or two groups of atoms can be brought
into closer proximity.
</P>
<P>The <I>move</I> style displaces the group of atoms by the specified 3d
distance.
</P>
<P>The <I>ramp</I> style displaces atoms a variable amount in one dimension
depending on the atom's coordinate in a (possibly) different
dimension.  For example, the second example command displaces atoms in
the x-direction an amount between 0.0 and 5.0 distance units.  Each
atom's displacement depends on the fractional distance its y
coordinate is between 2.0 and 20.5.  Atoms with y-coordinates outside
those bounds will be moved the minimum (0.0) or maximum (5.0) amount.
</P>
<P>The <I>random</I> style independently moves each atom in the group by a
random displacement, uniformly sampled from a value between -dx and
+dx in the x dimension, and similarly for y and z.  Random numbers are
used in such a way that the displacement of a particular atom is the
same, regardless of how many processors are being used.
</P>
<P>The <I>rotate</I> style rotates each atom in the group by the angle <I>theta</I>
around a rotation axis <I>R</I> = (Rx,Ry,Rz) that goes thru a point <I>P</I> =
(Px,Py,Pz).  The direction of rotation for the atoms around the
rotation axis is consistent with the right-hand rule: if your
right-hand's thumb points along <I>R</I>, then your fingers wrap around the
axis in the direction of positive theta.
</P>
<P>Distance units for displacements and the origin point of the <I>rotate</I>
style are determined by the setting of <I>box</I> or <I>lattice</I> for the
<I>units</I> keyword.  <I>Box</I> means distance units as defined by the
<A HREF = "units.html">units</A> command - e.g. Angstroms for <I>real</I> units.
<I>Lattice</I> means distance units are in lattice spacings.  The
<A HREF = "lattice.html">lattice</A> command must have been previously used to
define the lattice spacing.
</P>
<HR>

<P>IMPORTANT NOTE: Care should be taken not to move atoms on top of other
atoms.  After the move, atoms are remapped into the periodic
simulation box if needed, and any shrink-wrap boundary conditions (see
the <A HREF = "boundary.html">boundary</A> command) are enforced which may change
the box size.  Other than this effect, this command does not change
the size or shape of the simulation box.  See the
<A HREF = "change_box.html">change_box</A> command if that effect is desired.
</P>
<P>IMPORTANT NOTE: Atoms can be moved arbitrarily long distances by this
command.  If the simulation box is non-periodic and shrink-wrapped
(see the <A HREF = "boundary.html">boundary</A> command), this can change its size
or shape.  This is not a problem, except that the mapping of
processors to the simulation box is not changed by this command from
its initial 3d configuration; see the <A HREF = "processors.html">processors</A>
command.  Thus, if the box size/shape changes dramatically, the
mapping of processors to the simulation box may not end up as optimal
as the initial mapping attempted to be.
</P>
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<P><B>Restrictions:</B>
</P>
<P>You cannot rotate around any rotation vector except the z-axis for a
2d simulation.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "lattice.html">lattice</A>, <A HREF = "change_box.html">change_box</A>,
<A HREF = "fix_move.html">fix_move</A>
</P>
<P><B>Default:</B>
</P>
<P>The option defaults are units = lattice.
</P>
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